Exhaust gas emitted from internal combustion engines for automobiles and the like, for example, internal combustion engines such as gasoline engines or diesel engines, contain harmful components, such as carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and the like.
Therefore, an internal combustion engine is typically provided with an exhaust gas purification device for decomposing and removing these harmful components, and most of the harmful components are rendered innocuous by an exhaust gas purification catalyst mounted in the exhaust gas purification device. Well-known examples of the exhaust gas purification catalyst as described above include a three-way catalyst and a NOx storage-reduction catalyst.
A three-way catalyst is a catalyst in which the oxidation of CO and HC and the reduction of NOx are carried out simultaneously in a stoichiometric (theoretical air fuel ratio) atmosphere.
A NOx storage-reduction catalyst is a catalyst which oxidizes NO in an exhaust gas to NO2 and stores the NO2 in a lean atmosphere, and reduces the NO2 to nitrogen (N2) in a stoichiometric atmosphere and a rich atmosphere, effectively utilizing the changes in the exhaust gas components in the lean atmosphere, stoichiometric atmosphere, and rich atmospheres.
However, even with the use of these catalysts, the purification of exhaust gas still remains a problem, and various techniques have been examined.
Patent Document 1 discloses a composite metal colloid containing a plurality of metal elements, wherein the average particle diameter of the composite metal colloid is from 2 to 12 nm, and the plurality of metal elements are distributed substantially uniformly in the composite metal particles. Specifically, Patent Document 1 discloses a composite metal colloid dispersion obtained by mixing a solution of palladium chloride and a solution of rhodium chloride at a molar ratio of 1:1.
Patent Document 2 discloses an exhaust gas purification catalyst for removing CO or HC, wherein the catalyst includes an alloy containing Pd and Ag, and the alloy is supported on a carrier.